1. Field of the Invention
The present invention relates to a method of manufacturing a double pipe, in which an inner pipe is inserted into an outer pipe.
2. Description of Related Art
A double pipe used in an air conditioner is generally bent to have bending portions. The double pipe is bent by using a bender after a straight inner pipe is inserted into a straight outer pipe (e.g., JP-A-2004-245389, JP-A-2004-270916 corresponding to US 2004/0178627A1).
A manufacturing method of a double pipe having a bending portion is described in JP-A-2003-119844. In this method, a straight inner pipe is made longer than a straight outer pipe and is inserted into the straight outer pipe. Then, the inserted inner pipe and the outer pipe are bent at the same time without fixing its end parts, and the inner pipe is moved within a clearance range between both the inner pipe and the outer pipe in the bending portion so as to adjust the position of its end parts. However, in this case, because the bending is performed without fixing the end portions of both the outer and inner pipes, position error of the end portions becomes larger than a predetermined position range.
To overcome this problem, the end portions of the outer pipe and the inner pipe may be fixed during the bending. However, in this case, a large elastic energy is stored as residual stress in the inner pipe after the bending. Accordingly, when this double pipe is used for a vehicle air conditioner, the double pipe may be easily damaged due to a vibration of a compressor of the air conditioner or a vibration of the vehicle, etc.
The residual stress stored in the inner pipe may be removed after bending by performing an annealing process. However, in this case, the bent angle of the double pipe may be changed during the annealing, and manufacturing cost is increased due to the annealing process.